Project Abstract Alcohol tolerance refers to a diminished intensity of response as doses are repeated. Tolerance has long been implicated as a factor contributing to alcohol abuse and dependence by encouraging the use of escalating doses to reinstate initial effects of the drug. Alcohol is also well-known for its acute impairing effects on neurocognitive processes involved in the regulation of behavior and attention. Yet, little research has examined how such disturbances might reduce self-control over actual alcohol use, leading to patterns of abusive drinking (e.g., binge drinking). Moreover, despite interest in tolerance as a phenotypic marker for alcohol use disorders, little research has sought to determine how differences in alcohol sensitivity and tolerance might characterize populations at-risk for alcohol abuse, such as those with externalizing disorders (e.g., ADHD). The proposal is based on the working hypothesis that abuse potential of alcohol is determined by sensitivity to its reward-enhancing effects and to its disinhibiting effects. The proposed studies examine the contribution of tolerance to abuse potential as measured by changes in alcohol effects on basic mechanisms involved in the control and regulation of behavior. Studies will investigate: 1) the sensitivity of control mechanisms to the impairing effects of alcohol and their contribution to abuse potential; 2) tolerance development to the impairing effects on mechanisms that control behavior and attention; 3) the contribution of such tolerance to the abuse potential of alcohol; 4) the degree to which the inhibitory and attentional deficits associated with ADHD influence alcohol sensitivity and the development of tolerance; and 5) the degree to which the developmental course of tolerance is affected by concomitant use of stimulant drugs commonly used in the management of ADHD.